1. Field of the Invention
This invention relates to measuring and testing corrosion processes, and relates particularly to the instruments and electrochemical techniques used in the study of corrosion processes.
2. Description of the Prior Art
The determination of corrosion effects within metal pipelines conducting fluids, especially those under super atmospheric pressure, is becoming important not only from the standpoint of safety but also for extending the useful life of the fluid conveying systems. Many of the pipelines are relatively small in diameter (e.g. in the range of a few inches) but many gas interstate transmission lines are as large as 48 inches in diameter. These pipelines conduct fluids under pressures which range from less than a hundred psi to as high as 1400 psi from gas field production wells. The monitoring and prevention of corrosion effects in the nearly million miles of pipeline throughout the United States is relatively expensive and difficult. However, governmental bodies now are requiring adequate and reliable monitoring programs.
In certain pipelines, corrosion monitoring is a problem since the principal region of corrosion attack exists not within the bulk of the fluids carried within the pipeline but rather within a thin boundary liquid film which is disposed upon the interior sidewall surface of the pipeline. This film may be circumferential and extend over great lengths of the pipeline. However, the film may not cover the entire circumference of the interior sidewall and may be discontinuous or extend for only short lengths along the pipeline. Alternatively, a pipeline may have a film covering only the lower portion of the interior sidewall surface and discontinuously and periodic through any substantial length. Thus, corrosion monitoring of the film represents a severe problem since the film may not exist circumferentially, or continuously with time or pipeline length, but while present, it can be a highly active corrodent causing corrosion effects with severe pitting. Many corrosion monitoring devices have been proposed for determining the corrosion occurring within pipelines. None of the known corrosion monitoring systems, as will be described hereafter, have been found practical, accurate, or reliable relative to the thin boundary liquid film.
The thin boundary liquid film in a pipeline carrying gases usually consists of an aqueous phase, or mixed phases in which the aqueous phase is continuous. Also, a pipeline completely filled with a liquid can have a thin boundary liquid film developed through concentration effects, or velocity and shear stress effects, or as a function of hydraulics. As a result, the composition of the bulk fluid within the center of the pipeline is several magnitudes less corrosive in effect than the thin boundary liquid film along the interior sidewall surface of the pipeline. This is especially true where the fluid carried within the pipeline is comprised of mixed liquid phases such as water and hydrocarbons. Many pipeline transmissions involve liquids saturated with gases such as carbon dioxide and hydrogen sulfide. The concentration effects of these gases make the thin boundary liquid film extremely locally corrosive towards pitting of the interior sidewall surface of the pipeline. Under these circumstances, when an aqueous boundary liquid film contacts the pipe wall it is corroded. When no aqueous boundary liquid film exists corrosion ceases.
Known monitoring techniques which are capable of determining corrosion effects by fluids close to the interior sidewall surface of a pipeline may be divided into two general catagories. Disc-like coupons can be installed along the interior sidewall surface of the pipeline. The mounting arrangement of these coupons produces a discontinuity in the smooth sidewall of surface of the pipeline which disrupts the thin boundary liquid film which prevents an accurate determination of the corrosion effect, especially pitting. In addition, all coupon-like devices are long-term, direct measurement techniques. Thus, the intermittent presence of the thin boundary liquid film along the interior sidewall surface of a pipeline gives an indication over a long period of time that no corrosion effects existed. In actuality, the thin boundary liquid film, when it is present, creates very severe corrosion effects by pitting. For example, a high pressure gas pipeline was monitored using corrosion coupons that indicated over a 6 month period a corrosion attack that produced a coupon weight loss of only 3.86 milligrams. This weight loss correlated in the one inch square coupon to a corrosion rate of 0.06 mpy (mils per year). This pipeline was known to be covered with the thin liquid boundary film along its entire sidewall surface for only short periods of time. The actual failures of the pipeline indicated pitting in the range of 100 to 200 mpy which caused leaks in the operating interval of three to five years. Actual measurements in this pipeline by the Flush Mounted Probe Assembly of the present invention reflected that the thin boundary liquid film was present only approximately 2% of the time and produced an averaged long term corrosion rate reading of 3 mpy. However, instantaneous measurements with the Flush Mounted Probe Assembly during the presence of the film provided a readout of localized or pitting type of corrosion attack at rates of from 500 to 1400 mpy. Fortunately, inhibitors could be applied to reduce these corrosion rates by the same percentage as they do general corrosion and the effects in the pipeline were reduced by 90% so that the average long term measurement of corrosion through extended periods of time was 0.3 mpy, with a short period corrosion attack, as pitting, of 3 to 15 mpy as determined with the Flush Mounted Probe Assembly of the present invention.
Another technique for determining the effects of corrosion along the interior sidewall surface of the pipeline involves a flat-surfaced electrode or resistance wire, such as shown in U.S. Pat. No. 3,124,771. The flat resistance wire can be placed adjacent the interior sidewall surface of the pipeline. Then, the corrosion effects are determined by conventional resistance wire bridge-type instrumentation. As is well-known, resistance wire instrumentation provides neither instantaneous nor sensitive monitoring of corrosion effects. Such instrumentation involves a long term measurement of large changes in resistance produced by corrosion upon a sensing wire electrode surface. Thus, the resistance wire instrumentation, like the coupon, cannot measure the instantaneous presence of the rate of corrosion attack in the intermittent thin boundary liquid film which passes along the interior sidewall surface of a pipeline. No recognition was made in priorly used probe designs of the need to avoid introducing a surface discontinuity in the pipeline that disrupts the thin boundary liquid film present along the interior sidewall surface of the pipeline. The film usually has a thickness not over about 30 mils and is very fragile, being easily disrupted when the film is forming as under generating flow conditions. The surface discontinuity can be a protuberance or a crevice that induces sufficient macroscopic fluctuations in laminar flow to create turbulence sufficient to disrupt the film. In addition, no attempt was made to provide the electrode surfaces with a geometry and placement in a precise aligned relationship with the interior sidewall surface of the pipeline so that the thin boundary liquid film could be monitored directly.
The Flush Mounted Probe Assembly of the present invention provides both instantaneous and long term measurements of corrosion effect within the thin boundary liquid film disposed along the interior sidewall surface of a pipeline and is compatible with accepted electrochemical measurement instrumentation. Various types of polarization resistance type instrumentation may be employed but it is preferred to employ corrosion ratemeters such as are available commercially as Petrolite Instruments.
It is preferred for rapid and accurate results to measure the corrosion phenomena within a pipeline by employing electrochemical effects upon metal electrodes of a probe assembly positioned within the pipeline. An electrochemical process and apparatus especially useful in measuring corrosion rate is described in U.S. Pat. No. 3,406,101. In this technique, there is employed a corrosion ratemeter which includes a probe having three metal electrodes adapted to be exposed to a corrosive liquid. The instrumentation includes an adjustable current source, an ammeter, and a high impedance voltmeter as the primary components. The adjustable current source applies a small electric current between a "test" electrode and an "auxiliary" electrode. At the same time, the voltmeter monitors the induced polarization potential between the test electrode and a "reference" electrode. The current flow slightly polarizes the surface of the test electrode, and as a result, causes a shift in the potential between the test and reference electrodes. The current flow required to produce about 10 millivolts polarization is directly proportional to the corrosion rate of the test electrode undergoing corrosion.
One corrosion ratemeter employing the electrochemical technique which has found wide industrial acceptance is shown in U.S. Pat. No. 3,766,042. This corrosion ratemeter is portable and provides accurate instantaneous or long-term measurements of corrosion in conjunction with a probe having three metallic electrodes. Other corrosion ratemeters of similar manufacture for making corrosion measurements may be employed. It would be advantageous to employ these corrosion ratemeters with the present novel probe assembly for monitoring directly the corrosion occurring in corrodents residing in a high pressure gas environment within a pipeline. The Flush Mounted Probe Assembly of the present invention provides these results. This probe assembly permits the ready, leakproof and positive introduction of a multielectrode probe into the pipeline to be monitored in such a manner that the electrodes are precisely aligned in a contoured and diminsioned smooth end face at a selected relationship to the inner sidewall surface of the pipeline and the thin boundary liquid film is not disrupted. Normal operation of the pipeline is not disturbed. In addition, the probe is replaceably mounted within the pipeline and a fluid-tight seal is produced so that no pipeline fluids can leak past the probe. Other advantageous results obtained with the present probe assembly will be appreciated from the following discussion.